The present invention relates to a method and apparatus to detect multiple operating parameters in rolling element bearings. More particularly, the present invention relates to a method and apparatus to detect real time dynamic stress in rolling element bearings by way of an ultrasonic measurement system. The present invention further relates to a method and apparatus to detect rolling element bearing temperature and speed using the same ultrasonic measurement system.
Traditionally, rolling element bearing life has been predicted by applying a predetermined load to the rolling element bearing, and operating the bearing to a point of failure. Statistical averaging has then been used to predict bearing life during operating conditions. While the relationship between load and bearing life is important, other factors more closely relate to the operational life of rolling element bearing components.
Stress is the response of a material to an applied load over an area. Stress is therefore an internal reaction between elementary particles of a material in resisting separation, compaction, or sliding that tend to be induced by an external force, i.e. load. Total internal resisting forces are resultants of continuously distributed normal and parallel forces that are of varying magnitude and direction and are acting on elementary areas throughout the material. Stress may be identified as tensile, compressive, or shearing, according to the straining action.
An elastic material under stress will strain, i.e. deform, according to the formula:
stress=(strain)xc3x97(modulus of elasticity)
By way of example, for bearing steels, the modulus of elasticity is approximately 30 million pounds per square inch and strain is reported in inches of deformation per inch of initial size.
Prior techniques have been applied to the non-operational measurement of bearing parameters. For example, Womble et al. set forth in U.S. Pat. No. 4,763,523 that a pair of solid state transducer probes may be manually positioned by a technician to determine defects in railcar axle bearings. A crack which develops in a bearing race emits an acoustic shock pulse when a rolling element crosses the crack. The shock pulse is then detected by the solid state transducer probes. Likewise, Bourgeois-Jacquet, et al. set forth in EP 0 856 733 A1 that ultrasonic sensors may be inserted into an inner or outer ring of a crane bearing to detect surface and subsurface damage which result from crane operation. A technique for determining an amount of pre-load which has been applied to a bolt by using an electromagnetic acoustic transducer has also been described by Whaley et al. in U.S. Pat. No. 5,499,540.
Methods have also been used to infer a load zone in rolling element bearings from calibrated strain gauge measurements, such as described by Rhodes, et al., U.S. Pat. No. 5,952,587, which is incorporated herein by reference. However, these methods require modification of the bearing in the locations where support for the applied load is required. In other words, material from the bearing must be removed to provide for placement of strain gauges. Moreover, strain measurement techniques are dependent upon a number of factors that can not be readily predictable, such as clearances in the system and the strength of the supporting structure.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the invention, this is accomplished by dynamically detecting stress in rolling element bearings by transmitting and detecting an ultrasonic wave. Rolling element bearing stress is non-invasively and non-destructively measured in situ, i.e. during bearing operation. Only very minor modifications to the bearing element are required and no degradation of load-bearing capacity is made through the removal of rolling element bearing material. Ultrasonic stress measurement provides significant improvement over strain gauge measurement. Time changes of an acoustic signal are measured rather than resistance changes of a strain gauge, to thereby reduce the problems associated with environmental factors, such as temperature, and variation in the resistance of the measurement circuit.
Stress is measured in a rolling element bearing with an acoustic signal unit which transmits and receives an acoustic signal across the rolling element bearing. The time of flight of the acoustic signal is determined and a stress calculation unit calculates stress in the rolling element bearing from the time of flight. A first transducer generates an acoustic signal in the rolling element bearing in response to a first electrical signal while a second transducer generates a second electrical signal in response to the received acoustic signal. The first and second electrical signals are then compared to determine a difference in time, which corresponds to a change in stress in the bearing. Stress is calculated in a rolling element bearing according to the formula: stress=Kxc3x97Lxc3x97xcex94t where K is the acoustic velocity stress constant for the rolling element bearing, L is the distance traveled by the acoustic signal across the bearing, and xcex94t is the time of flight of the acoustic signal.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.